There are fields, chiefly that known as high-speed electronics, in which it is necessary to process detected pulsed message signals of very short duration which may recur at equally short intervals and whose initial shape may not be known but which nevertheless must be capable of being reconstituted with a certain degree of accuracy at the time of processing. This may be the case in the transmission of digital signals of the order of a hundred megabits with a very wide frequency band of the order of 500 MHz, for example.
In such transmission systems the received signals are converted into optical signals by means of high-speed optical emitters of the laser-diode type which enable wide-band links to be achieved whose band may range from approximately 100 Megahertz to 1 Gigahertz. The optical links are composed of optical fibers which, in comparison with electrical links, have the advantage of being electrically insulated and having a wide passband. However, such optical-fiber links do have the disadvantage of attenuating transmitted signals to an extent which may vary considerably in the course of transmission. This is due to the fact that the optical connectors employed are extremely sensitive to mechanical tolerances and to dust, and also to the fact that certain optical fibers of the multistrand type may have a varying number of their strands fractured, which attenuate still further the transmitted light. To these changes in level are added the changes caused by the transmitting and receiving members.
The transmitting member is a transducer which receives the signal to be processed from a sensor and converts it into an optical signal. In general, the transmitting member may be a solid-state laser or a light-emitting diode having a conversion characteristic for current to optical intensity which varies with time. The optical receiver which detects the signals transmitted by the fibers forming the optical link between the transmitter and the receiver may use either PIN diodes or avalanche diodes. PIN diodes are stable with temperature and as a function of the bias voltage. Avalanche photodiodes are approximately 100 times more sensitive to light but are extremely susceptible to variations in temperature and voltage.
The disadvantages of transmission systems employing optical links set against the advantages gained by using such links, as reviewed above, means that attention must be paid to the stability of the systems. In these systems, the transmitting and receiving elements are generally stabilized by amplitude-regulating loops. The variations in level which still occur are thus due almost exclusively to the optical links.
In conventional gain-control systems it is necessary to determine the losses suffered by signals in the optical link so as to restore the signal to the level it had on entering the link. The message signal to be processed, which may also be termed a useful signal, has superimposed on it a pilot signal of a predetermined level which lies outside the frequency band of the message signal. Since the attenuation to which the pilot signal is subjected is identical to that undergone by the message signal, it is possible at reception to restore the useful signal to the correct level.
On reception the pilot signal is extracted from the composite signal consisting of the message signal and the pilot signal and after detection controls a digitally operated variable-gain arrangement.
FIG. 1 is a diagram showing a prior-art arrangement of this kind. A sensor 1 receives the useful signal which, generally speaking, is of short duration and unknown level, and transmits it in electrical form to a modulator 3 which receives a pilot signal of fixed shape and amplitude from a source 2. The modulator combines the message signal Su and the pilot signal Sp and emits a composite signal Sc which is applied, for example, to a light-emitting diode 4 in series with a resistor 5. The diode converts the composite electrical signal Sc into an optical signal which is transmitted to the receiving part of the arrangement through an optical link 6. The output of the optical link feeds a receiving transducer, such as a photodiode 7, which is connected between a voltage source +V and ground via a resistor 8 and which again converts the composite optical signal into a composite electrical signal containing the useful signal and the pilot signal. Given that the attenuation undergone by the pilot signal can be assumed to be the same as that undergone by the message signal, the composite signal is applied to a fixed-gain amplifier 9 followed by a digital attenuator 15, this circuitry being connected to an automatic-gain-control arrangement which processes the extracted pilot signal and controls the attenuator in such a way that the message signal, which it emits either alone or still mixed with the pilot signal, is of the requisite level.
Starting from the fixed-gain amplifier 9, the automatic-gain-control arrangement comprises a band-pass filter 10 which separates the pilot signal, a non-clipping rectifier 11 which emits the pilot signal in the form of a D.C. voltage, a multibit analog-to-digital converter 12, the number of bits depending upon the accuracy required, a buffer register 13, and a code converter 14. The latter is connected to the attenuator 15 whose attenuation it adjusts. A high-pass filter 16 connected to the output of the attenuator 15 emits the message signal at an output 17. A timer or programmer 18 is provided to control the converter 12, the buffer register 13 and the code converter 14.
In the case of this prior-art arrangement, it is essential that the pilot or calibrated signal superimposed on the useful signal to produce the composite signal be situated outside the frequency band of the message signal. In many cases, this band extends from frequencies close to zero to frequencies which may be as high as several hundred Megahertz. In the prior art, the frequency of the pilot signal thus had to be set at a very low level, that is to say below the lowest message frequency, or at a very high level above the highest message frequency. In making this selection there was a danger of complicating the electronic circuitry and restricting the frequency band of the message signal either at the top or at the bottom.
The regulation of signal amplitudes by the automatic-gain-control circuit 10-14 associated with amplifier 9 and attenuator 15 in the load circuit of FIG. 1, in response to a pilot signal arriving over the fiber-optical link 6, has no effect upon the intensity of the luminous signal received by a photoelectrical transducer such as photodiode 7. It may therefore happen that this transducer becomes overloaded by large-amplitude optical signals which could have a destructive effect.